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Overview

A4 Power
Amplifier

Overview

Analogue

Control

Power Supply

Construction

Conclusion

Schematics

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As the stunningly original (!) name suggests, the A4 contains 4 separate
power amplifiers. This compact unit offers great flexibility - the following
modes of operation are available:

Four-channel 50 watts per channel operation for surround-sound or
multi-room operation.

Two-channel bi-amped mode, for suitable bi-wireable loudspeakers.

Two channel Bridged mode, offering around 150 watts per channel.

Combinations of the above are possible. For example, in a home theatre
application, rear speakers could be driven separately, while a centre
speaker could be bi-amped or bridged.

This amplifier uses four LM4780's working in inverted parallel mode. Each
of these offers around 50 watts into 8 ohms, but are capable of delivering
significant greater powers into lower impedances. A large custom-made torroidal
transformer forms the heart of the amplifier, designed with a low flux
density for reduced mechanical noise, and incorporating an electrostatic
screen to attenuate interference.

Operating mode selection

Each side of the amplifier has two power amplifiers and a mode switch
which selects from the following options:

DUAL

Each amplifier is completely separate.

BI-AMP

The signal applied to the top input socket is sent to both
amplifiers.

BRIDGED

Again, the signal applied to the top input socket is fed to both
amplifiers, but the lower amplifier inverts the signal. The negative
terminal of the loudspeaker is connected to the lower positive output
terminal, resulting in bridged operation. The output power is increased
to around 150 watts.

SINGLE

When a channel is not required, selecting this option will mute the
lower amplifier. This option helps ensures that the heat dissipation is
shared across both heatsinks in a stereo 50 watt per channel
application.

Control circuits

The microprocessor-based control circuits respond to the front panel switch,
the 12 volt trigger input, temperature sensors, a DC detection circuits and
mains supply failure. A soft-start circuit is incorporated to prevent large
inrush currents.

Front panel switch

The stainless steel front panel switch incorporates the blue power
LED. Brief presses will toggle between "on" and "standby" modes, and
holding the switch for more than half a second will mute the amplifier; this
is indicated by the power LED gently flashing. A brief press will cancel
this.

During Standby, the brightness of the red LED gently rises and falls,
indicating that the amplifier is happily "sleeping". While an interesting
programming exercise, the concept was shamelessly copied from Apple!

12 Volt trigger inputs

This amplifier has a 9 way D-type connector which is compatible with my
remote-controlled
preamp. A rear-mounted toggle switch determines whether the amplifier
should respond to the main or surround trigger signal (the preamp generates
the "main" signal when brought out of standby, and will additionally
generate the "surround" trigger when the multichannel source has been
selected). A rear-panel LED lights to indicate the presence of a 12 volt
trigger signal, which confirms the toggle switch is in the right position.
And while a 12 volt signal is present, the front panel switch still works as
normal. While this might seem like an obvious thing to say, it should be
realised that this is not always the case with similar products!

When mains power is applied to the amplifier, the status of the 12V input
is checked, and the amplifier is turned on if 12V is present. Otherwise, the
amplifier defaults to standby mode.

Supervisory and protection circuits

Should the amplifier overheat, the outputs are muted, and the front panel
power LED is flashed rapidly to indicate why the output has disappeared. This
flashing is quite distinct from the gentle rise and fall of the LED during
the user-selectable mute mode.

If the overtemperature condition persists for more than around 5 minutes,
the amplifier is shut down, and the red standby LED flashes an error code.
To recover from this state, a "hard reset" is required (switching the mains
off and on again) once the amplifier has cooled sufficiently.

The presence of a DC potential on any output will cause the amplifier to
be shut down immediately. As above, an error code will be displayed on the
standby LED, and the amplifier must be reset before operation can be
attempted.

The control circuits manage the power supply, and ensure that large inrush
currents are prevented at power-on (a common problem with audio power
amplifiers). Anti-cycle routines ensure that the amplifier isn't damaged by
being rapidly turned on and off. A rapid-action mains-failure detection
circuit is employed to ensure the control circuits are properly reset,
ensuring noise-free operation and freedom from "lock-ups" (always a
potential problem with microprocessor-based control circuits).

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© Copyright 2000-2016 Mark Hennessy. All Rights
Reserved. E&OE.
All opinions expressed are my own, and not those of my
employer.